kinetic andmechanistic study of the reduction of chromium(vi) by lactic...

Hindawi Publishing CorporationResearch Letters in Inorganic ChemistryVolume 2008, Article ID 314045, 4 pagesdoi:10.1155/2008/314045

Research Letter

Kinetic and Mechanistic Study of the Reduction ofChromium(VI) by Lactic Acid

Jinhuan Shan, Fang Wang, Changying Song, and Heye Wang

College of Chemistry and Environmental Science, Hebei University, Baoding 071002, Hebei Province, China

Correspondence should be addressed to Jinhuan Shan, [email protected]

Received 22 September 2008; Accepted 23 November 2008

Recommended by Rabindranath Mukherjee

The kinetics and mechanism of the reduction of chromium(VI) by lactic acid (Lac) in aqueous acidic medium was studied withspectrophotometry in a temperature range of 298.15 K∼313.15 K. Under the conditions of the pseudo-first order ([Lac]0 �[Cr(VI)]0), the observed rate constant (kobs) increased with the increase in [Lac] and [H+]. There is no salt effect. Based on theexperimental results, a probable reaction mechanism of oxidation was proposed. The rate equation derived from the mechanismcould explain all the experimental phenomena. Activation parameters along with rate constant of the rate-determining step havebeen evaluated.

Copyright © 2008 Jinhuan Shan et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Lactic acid is widely used in medications, foods, agriculture,industry, and so forth. It is a fine pH regulator and has animportance physiological function. Lactic acid can be usedto prepare degradable plastics and agroplastic film [1], whichcan reduce soil and water pollution.

Chromium exists usually in both trivalent and hex-avalent forms in aqueous systems. However, these twooxidation states are characterized by markedly differentphysical/chemical behavior and toxicity. Cr(III) is readilyprecipitated or absorbed on a variety of inorganic andorganic surfaces at near neutral pH [2, 3]. Cr(III), as anessential trace metal nutrient, also improves lipid profileand glucose tolerance by increasing the level of highdensity lipoprotein cholesterol and decreasing total serumcholesterol [4]. In contrast, the hexavalent form is of primeconcern because of its high toxicity. Cr(VI) usually occursas the highly soluble anion. The main sources of chromiumpollution are mining, leather tanning, cement industries,electro plating, production of steel and other metal alloys,photographic material, and corrosive paints [5–7]. Thehexavalent form of chromium has been demonstrated tobe associated with the toxic parameters and classified ashuman carcinogen and mutagen [8]. Breathing high levels ofhexavalent chromium can damage and irritate nose, lungs,

stomach, and intestine [9–11]. At present, there have beensome researches regarding this type of reaction system [12,13], while thorough research is essential for understandingits role in the oxidation of some organic compounds. Theobjectives of this research were to evaluate the kinetics ofchromate reduction by lactic acid and investigate their redoxmechanisms.

2. Experimental

2.1. Reagents and Apparatus

All chemicals used were of AR grade, and doubly distilledwater was used throughout the work. NaClO4 and HClO4

were used to adjust ionic strength and acidity of the reaction,respectively. The study was performed with a TU-1900 UV-Vis spectrophotometer fitted with a 501-thermostat (±0.1 K,Shanghai).

2.2. Kinetic Measurements andProduct Analysis

Under pseudo-first-order condition ([Lac]0 � [Cr(VI)]0),the reaction was initiated by mixing the Cr(VI) to lacticacid solution, and the kinetics was followed at 350 nm by

2 Research Letters in Inorganic Chemistry

700600500400300200

Wavelength (nm)

0

0.5

1

1.5

2

Abs

orba

nce

(a)

650600550500450

Wavelength (nm)

0

0.05

0.1

0.15

0.2

0.25

0.3

Abs

orba

nce

(b)

Figure 1: Spectra changes during the reduction of Cr(VI) by lactic acid at 298.15 K. [Cr(VI)] = 2.5 × 10−4 mol·L−1, [Lac] = 0.10 mol·L−1,[HClO4] = 0.50 mol·L−1, [DMSO] = 0.20 mol·L−1,�t = 150 seconds.

12108642

1/[H+] (mol·L−1)

50

100

150

200

250

300

350

400

1/k o

bs(s

)

313.15 K

308.15 K

303.15 K

298.15 K

Figure 2: Plots of 1/kobs versus 1/[H+]. [Cr(VI)] = 2.5 ×10−4 mol·L−1, [Lac] = 0.50 mol·L−1, [DMSO] = 0.20 mol·L−1,I = 1.0 mol·L−1.

monitoring decrease in absorbance due to Cr(VI) (Figure 1).The pseudo-first-order rate constants, kobs, were obtained asin the previous work [14]. The way for product analysis wassimilar to previous study [15], and the product was identifiedas the pyruvic acid and Cr(III) [16, 17].

3. Results and Discussions

3.1. Rate Dependence on [H+]

It was observed that the rate constants increased by increas-ing [H+], and the plots of 1/kobs versus 1/[H+]are linear withpositive intercept (Figure 2).

3.2. Rate Dependence on [Lac]

The influence of lactic acid on the rate of reaction was studiedat different temperatures. The plots of kobs versus [Lac]arestraight lines which pass the grid origin (Figure 3).

10.80.60.40.20

[Lac] (mol·L−1)

0

0.005

0.01

0.015

0.02

0.025

0.03k o

bs(s−1

)

313.15 K

308.15 K

303.15 K

298.15 K

Figure 3: Plots of kobs versus [Lac]. [Cr(VI)] = 2.5× 10−4 mol·L−1,[HClO4] = 0.50 mol·L−1, [DMSO] = 0.20 mol·L−1, I =1.0 mol·L−1.

3.3. Effect of Solvent and Ionic Strength (I)

The addition of solvent DMSO accelerates the rate ofreduction of Cr(VI) by lactic acid. When no catalyst or highconcentration of acid exists, the oxidation of DMSO is slowso it can be ignored [18–20]. The increase of ionic strengthof the reaction almost has no impact on the rate of reaction,indicating that there is no salt effect (Table 1).

3.4. Mechanism

The chromic acid also participates in acid-base equilibriumas follows [21]:

H2CrO4 � HCrO4− + H+, Ka1 = 1.8× 10−1,

HCrO4− � CrO4

2− + H+, Ka2 = 3.2× 10−7,

2HCrO4− � Cr2O7

2− + H2O, K = 33.

(1)

Research Letters in Inorganic Chemistry 3

Table 1: Effects of [DMSO]and ionic strength on kobs at 298.15 K.

[DMSO]/mol·L−1 0.050 0.10 0.20 0.30 0.40 0.50

103kobs/s−1 4.908 5.927 6.466 7.381 7.894 8.328

I/mol·L−1 0.60 0.80 1.0 1.1 1.2 —

103kobs/s−1 5.616 5.789 5.790 5.628 6.020 —

[Cr(VI)] = 2.5× 10−4 mol·L−1, [Lac] = 0.50 mol·L−1, and [HClO4] = 0.50 mol·L−1.

Hexavalent chromium exists primarily as chromic acid(H2CrO4), hydrogen chromate ion (HCrO4

−), and chromateion (CrO4

2−), depending on the values of pH. In acidicmedium of our experiment, dichromate exists predomi-nantly as monomer (HCrO4

−) and chromic acid (H2CrO4),and H2CrO4 plays a role of active species. Based on theexperimental results, we can learn that lactic acid participatesin the rate-determined step [22]. From the discussion above,the reaction mechanism was proposed as follows:

HCrO4− + H+ Kb� H2CrO4, (2)

H2CrO4 + CH3CHOHCOOH

k−→ Cr(IV) + CH3COCOOH + H2O,(3)

Cr(VI) + Cr(IV)fast−→ 2Cr(V), (4)

Cr(V) + CH3CHOHCOOH

fast−→ Cr(III) + CH3COCOOH + H2O.(5)

Here, reaction (3) is the rate-determined step. Based on themechanism above, the rate law is derived as follow:

−d[Cr(VI)

]T

dt= k

[H2CrO4

][Lac], (6)

d[Cr(VI)

]T

dt= kKb

[H+]

1+Kb[H+] [Lac]

[Cr(VI)

]T, (7)

kobs =kKb

[H+]

1+Kb[H+] [Lac], (8)

Kb = 1/Ka1, so kobs =k[H+]

Ka1 +[H+] [Lac], (9)

1kobs

= Ka1

k[Lac]· 1[H+] +

1k[Lac]

. (10)

Reaction (3) shows a redox decomposition with a two-electron transfer (C–H cleavage), and the redox productis pyruvic acid, which is consistent with the experimentalphenomena. From (8) and (10), the plots of kobs versus[Lac]are straight lines through the grid origin, and the plotsof 1/kobs versus 1/[H+] are linear with positive intercept. Therate constants of the rate determining step and activationparameters were obtained and presented in Table 2. Theeffect of solvent on reaction rate is an extremely complicated

Table 2: The rate constants and activation parameters for thereduction of Cr(VI) by lactic acid.

T/K 102k/mol−1·L·s−1 Thermodynamic activation

parameters (298.15 K)

298.15 4.74Ea = 30.98 kJ·mol−1

ΔS /= = −174.6 J·K−1·mol−1

ΔH /= = 28.50 kJ·mol−1

303.15 5.93

308.15 7.35

313.15 8.58

issue. We hold that the formation of H2CrO4 makes itselectron density drop. So, the reaction rate increases with thedecreasing of the solvent’s polarity [23]. This is consistentwith the experimental phenomena.

4. Conclusion

The kinetics of the reduction of chromium(VI) by lacticacid in the presence of DMSO has been studied. The activespecies of Cr(VI) is understood to be H2CrO4. Rate constantof the slow step and activation parameters were computed,respectively. The product of Cr(VI) oxidation of lactic acidis pyruvic acid, which suggested that the cleavage of C–H of lactic acid. Mechanism proposed for the reaction isin conformity with the product, mechanistic, and kineticstudies.

References

[1] H. Y. Wang, F. Li, S. P. Liang, and X. H. Chen, “Study onsynthesis and properties of biodegradable polylactic acid,”Chemistry & Bioengineering, vol. 22, no. 12, pp. 29–31, 2005.

[2] D. C. Schroeder and G. F. Lee, “Potential transformations ofchromium in natural waters,” Water, Air, & Soil Pollution, vol.4, no. 3-4, pp. 355–365, 1975.

[3] R. J. Kieber and G. R. Helz, “Indirect photoreduction of aque-ous chromium(VI),” Environmental Science & Technology, vol.26, no. 2, pp. 307–312, 1992.

[4] R. A. Anderson, “Chromium as an essential nutrient forhumans,” Regulatory Toxicology and Pharmacology, vol. 26, no.1, pp. S35–S41, 1997.

[5] N. K. Chandra Babu, K. Asma, A. Raghupathi, R. Venba,R. Ramesh, and S. Sadulla, “Screening of leather auxiliariesfor their role in toxic hexavalent chromium formation inleather—posing potential health hazards to the users,” Journalof Cleaner Production, vol. 13, no. 12, pp. 1189–1195, 2005.

[6] M. F. Bergamini, D. P. dos Santos, and M. V. B. Zanoni,“Development of a voltammetric sensor for chromium(VI)

4 Research Letters in Inorganic Chemistry

determination in wastewater sample,” Sensors and Actuators B,vol. 123, no. 2, pp. 902–908, 2007.

[7] S. Swarnalatha, T. Srinivasulu, M. Srimurali, and G. Sekaran,“Safe disposal of toxic chrome buffing dust generated fromleather industries,” Journal of Hazardous Materials, vol. 150,no. 2, pp. 290–299, 2008.

[8] H. Y. Shrivastava, T. Ravikumar, N. Shanmugasundaram, M.Babu, and B. U. Nair, “Cytotoxicity studies of chromium(III)complexes on human dermal fibroblasts,” Free Radical Biology& Medicine, vol. 38, no. 1, pp. 58–69, 2005.

[9] S. A. Katz and H. Salem, “The toxicology of chromium withrespect to its chemical speciation: a review,” Journal of AppliedToxicology, vol. 13, no. 3, pp. 217–224, 1993.

[10] E. Malkoc, Y. Nuhoglu, and Y. Abali, “Cr(VI) adsorption bywaste acorn of Quercus ithaburensis in fixed beds: prediction ofbreakthrough curves,” Chemical Engineering Journal, vol. 119,no. 1, pp. 61–68, 2006.

[11] P. Sanz, J. L. Moline, D. Sole, and J. Corbella, “Nasal septumperforation in chromate-producing industry in Spain,” Journalof Occupational Medicine, vol. 31, no. 12, pp. 1013–1014, 1989.

[12] H. Zeng and Z. Lin, “Kinetic study of 2,2′-bipyridyls catalyzedchromic acid oxidation of isopropanol,” Chinese Journal ofOrganic Chemistry, vol. 14, no. 4, pp. 526–531, 1994.

[13] B. L. Hiran, S. L. Chaplot, V. Joshi, and G. Chaturvedi,“Kinetics of the effect of some bidentate amino acid ligands inthe oxidation of lactic acid by chromium(VI)1,” Kinetics andCatalysis, vol. 43, no. 5, pp. 657–661, 2002.

[14] J.-H. Shan, J. Qian, M.-Z. Gao, S.-G. Shen, and H.-W. Sun,“Kinetics and mechanism of oxidation of n—propanolamineby dihydroxydiperiodatonickelate(IV) in alkaline medium,”Turkish Journal of Chemistry, vol. 28, no. 1, pp. 9–16, 2004.

[15] T.-S. Shi, J.-T. He, T.-H. Ding, and A.-Z. Wang, “Studies ofunusual oxidation states of transition metals III-kinetics andmechanism of oxidation of triethanolamine by diperiodatoar-genate(III) ion in aqueous alkaline medium,” InternationalJournal of Chemical Kinetics, vol. 23, no. 9, pp. 815–823, 1991.

[16] F. Feigl, Spot Tests in Organic Analysis, Elsevier, New York, NY,USA, 1956.

[17] Z. Khan, Z. A. Rafiquee, and K. Din, “Oxidation of L-methionine with aqueous chromic acid: a kinetic study,”Transition Metal Chemistry, vol. 22, no. 4, pp. 350–355, 1997.

[18] Z. Khan, K. Din, and M. Akram, “Kinetics and mechanismof the reduction of chromium(VI) with dimethyl sulfoxide,”Journal of Chemical Research Part S, no. 8, pp. 460–461, 1998.

[19] A. K. Das, S. K. Mondal, D. Kar, and M. Das, “Kineticsand mechanism of picolinic acid promoted chromium(VI)oxidation of dimethyl sulfoxide in the presence and absenceof surfactants,” Journal of Chemical Research Part S, no. 9, pp.574–575, 1998.

[20] B. Saha, M. Islam, and A. K. Das, “Kinetics and mecha-nism of 2,2′-bipyridine catalysed chromium(VI) oxidation ofdimethyl sulfoxide in the presence and absence of surfactants,”Journal of Chemical Research, vol. 2005, no. 7, pp. 471–474,2005.

[21] Dalian University of Technology Inorganic Chemistry Teach-ing and Research Room, Inorganic Chemistry, Higher Educa-tion Press, Beijing, China, 2001.

[22] J. J. Jin, Kinetics Principle of Chemical Reaction in Liquid Phase,Shanghai Science and Technology, Shanghai, China, 1984.

[23] X. C. Fu, W. X. Shen, and T. Y. Yao, Physical Chemistry,Advanced Education Press, Beijing, China, 1990.

Submit your manuscripts athttp://www.hindawi.com

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation http://www.hindawi.com Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttp://www.hindawi.com

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of

Hindawi Publishing Corporation http://www.hindawi.com Volume 2014


CatalystsJournal of

kinetic andmechanistic study of the reduction of chromium(vi) by lactic...

Documents